1. Field of the Invention
Our earlier applications as noted above were mainly concerned with the inhibition of corrosion of ferrous metals by certain described compositions which were active for such purpose when fully ionized, generally at an alkaline pH value of at least about 8.9. However, it was also disclosed therein that the same compositions, when at a relatively lower pH, were not only ineffective as corrosion inhibitors, but actually exhibited activity as corrosion agents. The utility of these compositions as metal cleaning agents based upon the mild corrosion ability of polyaspartic acid when at lower pH is now claimed herein.
The present invention relates to new and improved metal cleaning compositions, an unexpected and new use of biodegradable cleaning composition for ferrous metal and to improved processes for cleaning of ferrous metal surfaces susceptible to surface contamination. More particularly, this invention relates to processes for the use of metal cleaning polyamino acids effective to remove corrosion or adherent coating from ferrous metals conveniently and with environmentally friendly compositions.
2. Description of the Related Art
An important mechanism for cleaning metal involves the removal of surface deterioration and deposits and is achieved through the use of uniform corrosion rates. Unfortunately, certain common metal cleaning materials such as strong acids that are used widely as materials for metal cleaning agents have been found to be hazardous to public health and to the surrounding environment. Safe disposal of such hazardous is complicated and expensive. One such example is found in U.S. Pat. No. 3,847,663 to Shumaker. This patent discloses compositions supported by chelating agents such as ethylendiaminetetraacetic acid, trimethylenediaminetetraacetic acid, nitrilotriacetic acid and the like. In U.S. Pat. No. 4,470,920 to Leveskis, there is disclosed an aqueous solution containing nitric acid, sulfamic acid and an amino acid as a chelating agent.
Consequently, it has become desirable to examine the metal cleaning properties of biologically compatible and/or biodegradable compounds. Such compounds, if nontoxic, easy to produce in high purities, and biodegradable, can dramatically ease the chore of removal or recycling. Amino acids have been proposed for limited use. Aspartic acid is known to be inherently corrosive at slightly alkaline pH conditions. See K. Ramakrishnaiah, "Role of Some Biologically Important Compounds on the Corrosion of Mild Steel and Copper in Sodium Chloride Solutions", Bulletin of Electrochemistry, 2(1), 7-10 (1986). Therein it was disclosed that aspartic acid at a pH of 8 actually accelerated corrosion. In fact, even when combined with an excellent corrosion inhibitor for mild steel such as papaverine, the presence of aspartic acid maintained the solution's corrosiveness.
The thermal condensation of alpha amino acids to form polymers with loss of water has been known for many years. Early interest in such processes related to theories for formation of prebiotic polypeptides. For the purpose of testing such theories laboratory experiments used powdered L-aspartic acid, usually packed in the bottom of a flask which was then heated below the melting point of the acid. Such reactions were slow and took place over many hours. One such example is reported by Kokufuta et al. in Bulletin of the Chemical Society of Japan Vol. 51 (5) 1555-1556 (1978) "Temperature Effect on the Molecular Weight and the Optical Purity of Anhydropolyaspartic Acid Prepared by Thermal Polycondensation." The structure of anhydropolyaspartic acid has been thoroughly investigated such as by J. Kovacs et al. in J.O.C.S. Vol. 26 1084-1091 (1961).
In recent years many utilities have been suggested for anhydropolyamino acid. Such polyamides have been suggested as potential drug carriers by Neuse et al. in Die Angewandte Makronmolekulare Chemie 192 35-50 (1991) "Water-soluble polyamides as potential drug carriers." They have also been tested as scale inhibitors with respect to natural sea water and calcium sulfate in particular by Sarig et al. as reported by the National Council on Research and Development (NRCD 8-76, Seawater Desalination 150-157 (1977). Polyaspartic acid has been well known for its ability to disperse solid particles in detergent formulations, having been mentioned as a dispersant in numerous patents, a few of which are U.S. Pat. Nos. 4,363,797; 4,333,844; 4,407,722 and 4,428,749. As a departure from the usual manner of utilizing polyaspartic acid in detergent formulations it is reported in Australian Patent A-14775/92 that the polyamide is added to the wash liquor which, upon hydrolysis in situ, is converted into a biodegradable polypeptide builder. Also, as described in U.S. Pat. No. 4,971,724 to Kalota et al., it has been discovered that compositions comprising polyamino acids such as aspartic acid, when ionized at alkaline pH, effectively inhibit corrosion of ferrous metals in the presence of aqueous medium. Various derivatives of polyamino acids have also been made wherein attributes have been supplied by groups attached to reactive sites on the molecule. One such example is disclosed in U.S. Pat. No. 3,846,380 to Fujimoto et al.
Because of the various impending potential utilities of anhydropolyamino acids, interest in processes for preparing such compounds in large volume, particularly polyaspartic acid, has increased. This interest has resulted in several recent patents being issued which are directed to fluid bed systems; in particular, U.S. Pat. No. 5,219,986 to Cassata. Other such patents are U.S. Pat. No. 5,057,597 and 5,221,733 to Koskan and Koskan et al. respectively.
A process for the cleaning of metals of various types by polyamino acids having an additional carboxyl groups (such as polyaspartic acid) under conditions wherein such amino acids are at lower pH would represent a surprisingly unexpected discovery while satisfying a long-felt need for a safe, biodegradable yet effective cleaner in the industry.